This invention relates to a method and apparatus for coating a web of reconstituted tobacco, and particularly to a method and apparatus for coating both sides of a web of reconstituted tobacco without the web having to travel unsupported at any point during coating.
Reconstituted tobacco is typically made by recovering small fragments and particles of tobacco and tobacco dust which are generated during tobacco processing and forming them into a sheet or web. A web of reconstituted tobacco can be made by forming a liquid suspension of the tobacco particles, and then forming the sheet by casting the suspension on a solid substrate, or depositing it on a screen or "wire" such as is used in the Fourdrinier paper-making process, and allowing the liquid to drain off. When the remaining solids are dried, a sheet of tobacco similar to paper results.
The tobacco "paper" is low in flavor and other subjective components. Many of the subjective components are dissolved in the liquid used to make the sheet. The sheet is therefore customarily "sized" by saturation in a bath of "size"--i.e., a liquor of tobacco solubles--which are frequently recovered at least in part from the sheet-making process.
When very fine tobacco particles and tobacco dust are used in the sheet-making process, particularly in the Fourdrinier-type process, they may pass through the screen with the liquid and not remain in the sheet. Alternatively, they may become caught in the screen, preventing liquid from draining and reducing the product capacity of the equipment.
Therefore, it is known to mix very fine tobacco particles and tobacco dust with a liquid--e.g., the sizing liquor--to form a slurry and then to apply the slurry as a coating to one or both sides of a reconstituted tobacco sheet. Although the fine particles and dust do not significantly improve the subjective qualities of the reconstituted tobacco sheet, coating them as a slurry allows their volume to be added to the sheet without fouling the sheet-making equipment. Such a process is disclosed in commonly-assigned U.S. Pat. No. 4,421,126, in which it is taught that the slurry can be applied by a blade coater.
Reconstituted tobacco sheet has a relatively low wet strength. For that reason, when it has been desired in the past to coat both sides of a sheet of reconstituted tobacco, the practice has been to coat one side, dry the sheet, coat the other side, and again dry the sheet. Furthermore, it has been necessary to support the wet coated sheet as it passes through any open draws in the equipment to prevent it from tearing under its own weight. The known equipment for coating both sides of a reconstituted tobacco sheet therefore takes up relatively large amounts of space, operates relatively slowly and consumes relatively large amounts of energy, particularly for drying.